Sarah Whyte and Fergus Hunter

The Abbott government's failed attempt to return 157 asylum seekers to India cost taxpayers more than $12 million, as passengers were kept captive on the high seas for nearly a month before being brought to the Australian mainland.

A cost analysis by Fairfax Media has revealed as much as $12,020,778 was outlayed by the government to ensure the 157 Sri Lankan nationals did not arrive on Australian shores to challenge Immigration Minister Scott Morrison's resolve of "stopping the boats".

The government has consistently claimed it will save $2.5 billion by stopping the boats, by shutting down nine onshore detention centres and dramatically reducing the number of asylum seekers arriving by boat.

The cost of keeping the asylum seekers at sea, based largely on Senate Estimates figures, includes the daily running cost of the navy frigate HMAS Perth which first intercepted the boat that had travelled from India, and the customs vessel Ocean Protector where the asylum seekers were kept on board for 29 days.

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It also includes the cost of accommodation at Cocos Island for 14 immigration officials and Curtin detention centre for the 157 asylum seekers; legal costs associated with the High Court challenge that was launched against the government; and Mr Morrison's last-minute trip to New Delhi where he presented Indian officials with two cricket bats worth $695 each.

The Cocos Beach Motel confirmed 14 immigration officials had stayed on the island over three days in July, costing $5345, while the transferral of the asylum seekers onto three chartered flights from Cocos Islands to Curtin would have cost the government $600,000 based on 2012 Senate Estimates.

The five-star Taj Mahal Hotel in New Delhi also confirmed Mr Morrison had stayed there on July 21 as he met Indian officials to request the country take back the asylum seekers who had left from the south of India in early June.

For the government to then defend the High Court case, where lawyers had launched an injunction to stop the boat being sent back to India, it would have cost around $136,500.

This includes "upper band" legal fees of a junior counsel and QC Stephen Donaghue who charges about $7000 a day and spent 12 days preparing and three days appearing at trial. This cost does not include the team of lawyers from the Australian Government Solicitor service who also worked on the case.

Until the court case, Mr Morrison had refused to acknowledge the existence of the boat, citing "on water" matters.

"The Australian people have still not received a decent explanation from the Immigration Minister as to why he kept 157 people at sea for close to a month," Mr Marles said.

"He went to India armed with cricket bats and came back with what he called a 'landmark' agreement that was in tatters before it even commenced."

During the 29 days, asylum seekers, who first spoke to Fairfax Media on June 27, were separated into gender groups and placed in windowless rooms where they were given only three hours sunlight a day, according to court documents.

Once India had rejected Australia's plea to return the asylum seekers there, Mr Morrison brought the group to Curtin detention centre in Western Australia before transferring them to the offshore detention centre in Nauru five days later.

Human Rights Watch said the cost was "outrageous and unnecessary".

"A cost-effective and rights-respecting response would bring asylum seekers onshore in Australia and process their claims properly and quickly, without detaining them longer than necessary," HRW president Elaine Pearson said.

Greens immigration spokeswoman Sarah Hanson-Young said the financial cost could have been entirely avoided if the asylum seekers had been taken straight to Christmas Island.